Trivalent Actinide Ions Showing Tenfold Coordination
in Solution
Posted on 2020-08-14 - 16:34
Trivalent actinides generally exhibit
ninefold coordination in
solution. 2,6-Bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (nPr-BTP),
a tridentate nitrogen donor ligand, is known to form ninefold coordinated
1:3 complexes, [An(nPr-BTP)3]3+ (An = U, Pu,
Am, Cm) in solution. We report a Cm(III) complex with tenfold coordination
in solution, [Cm(nPr-BTP)3(NO3)]2+. This species was identified using time-resolved laser fluorescence
spectroscopy (TRLFS), vibronic side band spectroscopy (VSBS), X-ray
photoelectron spectroscopy (XPS), and density functional theory (DFT).
Adding nitrate to a solution of the [Cm(nPr-BTP)3]3+ complex in 2-propanol shifts
the Cm(III) emission band from 613.1 to 617.3 nm. This bathochromic
shift is due to a higher coordination number of the Cm(III) ion in
solution, in agreement with the formation of the [Cm(nPr-BTP)3(NO3)]2+ complex. The formation of this
complex exhibits slow kinetics in the range of 5 to 12 days, depending
on the water content of the solvent. Formation of a complex [Cm(nPr-BTP)3(X)]2+ was not
observed for anions other than nitrate (X– = NO2–, CN–, or OTf–). The formation of the [Cm(nPr-BTP)3(NO3)]2+ complex was studied as a function
of NO3– and nPr-BTP concentrations, and
slope analyses confirmed the addition of one nitrate anion to the
[Cm(nPr-BTP)3]3+ complex. Experiments with varied
nPr-BTP concentration show that [Cm(nPr-BTP)3(NO3)]2+ only forms at nPr-BTP
concentrations below 10–4 mol/L whereas for concentrations
greater than 10–4 mol/L the formation of the tenfold
species is suppressed and [Cm(nPr-BTP)3]3+ is
the only species present. The presence of the tenfold coordinated
complex is supported by VSBS, XPS, and DFT calculations. The vibronic
side band of the [Cm(nPr-BTP)3(NO3)]2+ complex exhibits a nitrate stretching
mode not observed in the [Cm(nPr-BTP)3]3+ complex.
Moreover, XPS on [M(nPr-BTP)3(NO3)](NO3)2 (M = Eu, Am) yields
signals from both non-coordinated and coordinated nitrate. Finally,
DFT calculations reveal that the energetically most favored structure
is obtained if the nitrate is positioned on the C2 axis of the D3 symmetrical
[Cm(nPr-BTP)3]3+ complex with a bond distance
of 413 pm. Combining results from TRLFS, VSBS, XPS, and DFT provides
sound evidence for a unique tenfold coordinated Cm(III) complex in
solutiona novelty in An(III) solution chemistry.
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